Multigenerational reproductive toxicology study of ethinyl estradiol (CAS No. 57-63-6) in Sprague-Dawley rats.

Background: Ethinyl estradiol is a potent synthetic estrogen that is widely prescribed in oral contraceptives and is also used in the treatment of breast and prostate cancer. Ethinyl estradiol is one of a class of chemicals known as“environmental estrogens” which can affect the hormone activities and possibly reproductive function of wildlife and humans through exposure. The NTP conducted a series of studies on three such chemicals to detect if exposure over the course of multiple generations could have any cumulative effect on animals' reproductive systems or development of cancers. This report describes the results of a set of studies in which rats and their offspring were exposed to ethinyl estradiol over the course of four generations.

Methods: The continuous-breeding study began with groups of 35 Sprague-Dawley rats of each sex exposed to ethinylestradiol in their feed at concentrations of 2, 10, or 50 parts per billion (ppb). Control animals received the same feed with no ethinyl estradiol added. Animals from the same dose treatment groups were paired and mated, and 25 litters of pups at each exposure concentration (culled to four males and four females each) were continued on study and given feed containing the same concentration of ethinyl estradiol. The process was repeated through a second and third generation, after which the pups were given control feed only, and two more generations were bred in the same manner and given control feed without ethinyl estradiol. Measures of fertility and reproduction were taken for each generation and tissues from the study animals were examined histopathologically.

Results: In all three offspring generations the time to vaginal opening (a measure of onset of puberty) was accelerated in females fed 50 ppb ethinyl estradiol. In the first two offspring generations the estrous cycles of the exposed females were prolonged or aberrant prior to mating. Male rats exposed to ethinyl estradiol had increased rates of mammary gland hyperplasia and mineralization of the kidney tubules.

Conclusions: We conclude that exposure to trace amounts of ethinyl estradiol in the feed showed clear biological activity in male and female rats, including reduced body weights in both sexes, perturbed estrous cycles in females, and induction of mammary gland hyperplasia and kidney tubule mineralization in males.